/Engine/Source/Editor/EditorFramework/Public/Toolkits/IToolkitHost.h
Notificaton when the active viewport changed
typedef TMulticastDelegate_TwoParams< void, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [IAssetViewport](API\Editor\UnrealEd\IAssetViewport) >, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [IAssetViewport](API\Editor\UnrealEd\IAssetViewport) > > FOnActiveViewportChanged